Fiber bed mist eliminators have wide industrial application in the removal of aerosols from gas streams. The generation of aerosols (“mist”) in gas streams is common in the course of manufacturing processes. Aerosols can be formed, for instance, as a result of mechanical forces (e.g., when a flow including a liquid runs into a structure) that atomize a liquid. Cooling of a gas stream may result in the condensation of vapor to form a mist, and chemical reactions of two or more gases may take place at temperatures and pressures where the reaction products are mists. However the aerosol comes to be in the gas stream, it can be undesirable to inject the aerosol into other processing equipment because the aerosol may be corrosive or otherwise lead to damage or fouling of the processing equipment. Moreover, it can be undesirable to exhaust certain aerosols to the environment. Some of the more frequent applications of fiber bed mist eliminators include removal of acid mists, such as sulfuric acid mists, in acid manufacturing, removal of plasticizer mists in the manufacture of polyvinyl chloride floor or wall coverings and removal of water-soluble solid aerosols from the emissions of ammonium nitrate prill towers. In these various applications, fiber bed mist eliminators may achieve separation efficiencies of 99% or greater depending upon, among other things, the thickness of the fiber bed.
It is generally known that fibers made of various materials may be used to construct fiber beds for fiber bed mist eliminators. The fiber bed is designed to collect fine liquid mist and soluble solid particles entrained in a moving gas stream and drain them through the structure of the bed. Typically, beds of collecting fibers are associated with metal wire screens or similar external support structures. The combination of a bed of collecting fibers and external support structure is known as a fiber bed assembly. As used hereinafter, fiber bed refers to that portion of the fiber bed assembly apart from any such support structure. Fiber beds may be formed by packing bulk fiber between two opposing support screens (bulk-packed beds), pre-forming a tube of fiber bed material, or winding a roving made of fibers around a cylindrical support screen (wound beds). Although not limited to such a configuration, fiber bed assemblies are most often configured in the form of a vertical cylinder. Cylindrical fiber bed assemblies permit a high effective fiber bed surface area in a minimum of space.
In operation, a horizontal stream of gas containing a liquid and/or wetted soluble solid aerosol is made to penetrate and pass through the fiber bed of the fiber bed assembly. The fibers in the fiber bed capture the aerosol in the gas by the mechanisms of impaction, interception, and Brownian diffusion. The captured aerosol coalesces on the fibers to form droplets of liquid in the fiber bed. The moving gas urges the droplets to move toward the downstream face of the fiber bed where the captured liquid exits the fiber bed and drains downward under the force of gravity.
The fibers which make up the fiber bed may be made from a variety of materials. Materials utilized to make bed fiber include, for example, metals such as stainless steel, titanium, etc., fibers of polymeric materials such as polyesters, polyvinylchloride, polyethylene terphthalate, nylons, polyethylene, polypropylene etc., and glass. In applications where corrosive conditions and/or high temperatures are encountered, long staple, chemical grade glass fibers have found particularly widespread use in fiber beds of fiber bed mist eliminators. Fibers ranging in diameter from 5 microns or less to more than 200 microns, as well as combinations of fibers of varying diameters, have been used in fiber beds. The bulk density of prior art fiber beds ranges from about 5 lb/ft3 (80 kg/m3) to greater than 20 lb/ft3 (320 kg/m3), while fiber bed thickness ranges from about 0.5 to about 6 inches (1 to 15 cm) or more, depending upon the desired separation efficiency.
Re-entrainment of collected liquid from the downstream surface of the fiber bed often causes problems. These problems can include any of the following individually or in combination; fouling of downstream process equipment, degradation of product purity, corrosion to ductwork and in some cases difficulty in achieving emission requirements. Re-entrainment in fiber bed separators can arise from two mechanisms. As the liquid drains down through the fiber bed and/or the downstream surface thereof, the moving gas stream can cause some of the draining liquid to break or bubble out of the descending liquid stream and become re-entrained in the gas stream as droplets. This problem is particularly severe at the bottom of a vertically disposed fiber bed since all of the liquid collected by the fiber bed necessarily drains to the bottom and from a practical standpoint because of gas phase drag on the liquid, out the downstream surface at the bottom of the fiber bed. At this disengagement point where the greatest cumulative drainage occurs, gas phase drag can cause bubbling, “spitting”, jetting or fragmentation of the draining liquid. As these bubbles break, large to sub-micron sized fragments or droplets are formed which are carried away by the moving gas stream as what is termed “bubble re-entrainment”. For example, droplets formed by fragmentation or bubble bursting which could become re-entrained may have a size ranging from 2 to 2,500 microns.
The second re-entrainment mechanism termed “bed re-entrainment” occurs at gas bed velocities so high that gas phase drag on the draining liquid in the entire fiber bed on downstream discharge surfaces of the fiber bed causes bubbling, spitting, jetting and fragmentation into re-entrainment. Thus, in a given fiber bed and at a constant liquid loading, as bed velocity increases, a point is reached where bubble re-entrainment begins. This first occurs at the bottom of the fiber bed on the gas discharge surface of the collecting media. As the bed velocity is increased even further re-entrainment begins to occur at higher levels on the fiber bed until with only minor increases in velocity, re-entrainment is occurring from substantially the entire gas discharge surface of the fiber bed. This is typically referred to as a totally flooded condition.